1. Field of the Invention
The present invention relates to the field of induction resistivity measurements for evaluation of potential hydrocarbon bearing formations using downhole induction instruments having reinforced metal mandrel (monitoring while drillingxe2x80x94MWD) or significant amounts of electrically conductive part parts in or around the sensor section. The present invention provides for compensation of eddy currents induced in the tool body which create additive error signals that interfere with resistivity measurements.
2. Summary of the Related Art
Electromagnetic induction resistivity well logging instruments are well known in the art. These induction resistivity well logging instruments are used to determine the electrical conductivity, or its converse, resistivity, of earth formations penetrated by a borehole. Formation conductivity has been determined based on results of measuring the magnetic field due to eddy currents that the instrument induces in the formation adjoining the borehole. The electrical conductivity is used for, among other reasons, inferring the fluid content of the earth formations. Typically, lower conductivity (higher resistivity) is associated with hydrocarbon-bearing earth formations. The physical principles of electromagnetic induction well logging are well described, for example, in, J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists, December 1962. Many improvements and modifications to electromagnetic induction resistivity instruments described in the Moran and Kunz reference, supra, have been devised, some of which are described, for example, in U.S. Pat. No. 4,837,517 issued to Barber, in U.S. Pat. No. 5,157,605 issued to Chandler et a. and in U.S. Pat. No. 5,600,246 issued to Fanini et al.
The conventional geophysical induction resistivity well logging tool is a probe suitable for lowering into the borehole and it comprises a sensor section and other, primarily electrical, equipment for acquiring the data to determine the physical parameters that characterize the formation. The sensor section, or mandrel, comprises induction transmitters and receivers positioned along the instrument axis and arranged in the order according to particular instrument specifications. The electrical equipment generates an electrical voltage to be further applied to a transmitter induction coil, conditions the signals coming from receiver induction coils, processes the acquired information. The acquired data then has been stored or by means of telemetry sent to the earth""s surface through a wire line cable used to lower the tool into the borehole.
In general, when using a conventional induction logging tool with transmitters and receivers (induction coils) having their magnetic moments oriented only along the borehole axis, the hydrocarbon-bearing zones are difficult to detect when they occur in multi-layered or laminated reservoirs. These reservoirs usually consist of thin alternating layers of shale and sand and, oftentimes, the layers are so thin that due to the insufficient resolution of the conventional logging tool they cannot be detected individually. In this case the average conductivity of the formation is evaluated.
Thus, in a vertical borehole, a conventional induction logging tool with transmitters and receivers (induction coils) oriented only along the borehole axis responds to the average conductivity that combines the conductivity of both sand and shale. These average readings are usually dominated by the relatively higher conductivity of the shale layers. To address this problem, loggers have turned to using transverse induction logging tools where transmitters and receivers (induction coils) have their magnetic moments oriented transversely with respect to the tool longitudinal axis. The instrument for transverse induction well logging was described in U.S. Pat. No. 5,781,436 issued to Forgang et al.
In the transverse induction logging tools the response of transversal coil arrays is also determined by an average conductivity, however, the relatively lower conductivity of hydrocarbon-bearing sand layers dominates in this estimation. In general, the volume of shale/sand in the formation can be determined from gamma-ray or nuclear well logging measurements. Then a combination of the conventional induction logging tool with the transversal induction logging tool can be used for determining the conductivity of individual shale and sand layers.
One of the main difficulties in interpreting the data acquired by a transversal induction logging tool is associated with its response vulnerability of to borehole conditions. Among these conditions there are a presence of a conductive well fluid as well as well bore fluid invasion effects. A known method for reducing these unwanted impacts on a transversal induction logging tool response was disclosed in L. A. Tabarovsky and M. I. Epov, Geometric and Frequency Focusing in Exploration of Anisotropy Seams, Nauka, USSR Academy of Science, Siberian Division, Novosibirsk, pp. 67-129 (1972) and L. A. Tabarovsky and M. I. Epov, Radial Characteristics of Induction Focusing Probes with Transverse Detectors in an Anisotropic Medium, Soviet Geology And Geophysics, 20 (1979), pp. 81-90.
The known method has used a transversal induction logging tool comprising induction transmitters and receiver (coils). By irradiating a magnetic field the induction transmitter induces currents in the formation adjoining the borehole; in turn, the receivers measure a responding magnetic field due to these currents. To enable a wide range of vertical resolution and effective suppression of the unwanted borehole effects, measurements of magnetic field form the formation can be obtained at different distances between the transmitter and receiver. However, even with these modifications, the data logs obtained with a conventional transversal induction logging instruments can be inconsistent, difficult to interpret, and the problems have been exacerbated while logging through a sequence of layers.
In the induction logging instruments the acquired data quality depends primarily on the electromagnetic environment in which the tool operates. Thus, in an ideal case, the logging tool measures signals induced only by eddy currents excited in the formation by the primary magnetic field of the induction transmitter. Variations in the magnitude and phase of the eddy currents occurring in response to variations in the formation conductivity are reflected as respective variations in the output voltage of induction receivers. In conventional induction instruments these receiver voltages are signal conditioned and then processed using analog phase sensitive detectors or, primarily, algorithmically. The processing allows for determining both receiver voltage amplitude and phase with respect to the induction transmitter current or its magnetic field waveform.
As in an open hole induction logging, the induction instruments currently deployed in monitoring while drilling (MWD) operations typically utilize solenoid-type transmitter and receiver induction coils coaxial with the tool mandrel. These instruments produce a xe2x80x9cclassicalxe2x80x9d set of induction measurements in the propagation mode, measuring attenuation and phase shift in the transmitted magnetic field due to the influence of the adjacent formation. Known induction tools utilize two coaxial receiver coils positioned in the center of the mandrel and two sets of balanced transmitter coils on both sides of the receiver coils. This balanced coil configuration, when operating at two frequencies of 400 kHz and 2 MHz, enables the typical known induction instrument to quantitatively evaluate eight directional horizontal formation resistivities for a plurality of investigation depths.
As a general rule for induction tool design, a high degree of magnetic coupling between the tool transmitter and the formation and the tool receiver and the formation is desirable. This high magnetic coupling between the tool transmitter/receiver and the formation facilitates increased instrument efficiency and increased overall signal to noise ratio. However, the presence of conductive bodies typically found in the mandrel of known induction instruments becomes problematic, resulting in appearance of an additional and unavoidable magnetic coupling between these bodies and different sources of electromagnetic radiation reception. Primarily, parasitic magnetic coupling between the transmitter, receiver and formation from one side and the conductive metal parts of the downhole tool from another side creates problems associated with parasitic eddy currents induced on the tool body surface.
The parasitic eddy currents flowing on the surface of the downhole tool produce undesirable magnetic fields that interact with resistivity measurements as additive error components. These induced magnetic fields reduce the overall transmitter moment, couple directly into the receiver coil(s) and create undesirable offsets in measurement signals. Moreover, eddy currents are temperature and frequency dependent that makes their adverse effects difficult, if not impossible to account and compensate for by known methods. Thus, there is a need for a method and apparatus that compensates for the adverse effects of eddy currents.
The present invention provides improved measurement capabilities for induction tools for formation resistivity evaluations. The present invention provides electromagnetic transmitters and sensors suitable for transmitting and receiving magnetic fields in radial directions that are orthogonal to the tool""s longitudinal axis with minimal susceptibility to errors associated with parasitic eddy currents induced in the metal components surrounding the transmitter and receiver coils. The present invention provides increased effective tool surface impedance by increasing self-inductance of the paths in which induced eddy currents flow on the surface of the multi-component induction instruments.
The present invention enables downhole tool designers to build more effective and better-protected radial induction arrays for existing and future downhole instruments operating in the frequency and/or time domains. In this case the array measurement results contain information primarily about the formation""s vertical resistivity. The present invention makes it possible to combine radial arrays with coaxial arrays that conventionally measure horizontal formation resistivity. This combination enables obtaining a full resistivity tensor to evaluate formation resistivity anisotropy.
In one aspect, the present invention provides a composite non-conductive housing to reduce or even avoid the effects of parasitic eddy currents flowing on the tool surface. In another aspect, the present invention provides a non-magnetic housing that is conductive which reduces the effects of conductive materials near coils and, primarily, the receiver. In another aspect of the invention, a non-conductive coating is placed over the housing to prevent high frequency eddy currents from leaking from the housing in the conductive mud of the adjacent wellbore and returning to the housing.